Use of spike tires has been banned by law in Japan so as to prevent powder dust pollution resulting from the spike tires, and thus studless tires are now used instead of the spike tires in cold regions. In order to improve grip performance of the studless tires on ice or snow, there is a method that decreases an elastic modulus at low temperatures and improves traction. Particularly, the braking force on ice is largely affected by an effective-contact area between rubber and ice. In order to enlarge the effective-contact area, a rubber has been desired which is flexible at low temperatures.
On the other hand, where the hardness of rubber alone is decreased using a method that comprises, for example, increasing the amount of oil, there is a problem that the handling stability decreases.
Generally, a natural rubber or a butadiene rubber is often used as a main ingredient in a tread rubber of a studless tire (for example, see JP 2007-176417 A). This is because these rubbers have a low glass transition temperature and flexibility even though they have high strength. However, reversion occurs when the natural rubber or butadiene rubber is sulfur-vulcanized. In this phenomenon, rubber degrades or its cross-linked state deteriorates, so that the elastic modulus at low temperatures also decreases. However, the inventors of the present invention have found from their studies that the hardness also excessively decreases and thus the handling stability and the abrasion resistance decrease. In addition, the reversion may increase tan δ at high temperatures excessively, thereby decreasing fuel economy.
For some tires including studless tires, vulcanization is performed at higher temperatures in order to increase the productivity of tires. In this case, the above phenomenon, particularly, is more pronounced. Accordingly, there is an additional problem of decrease in abrasion resistance caused by the reversion.
In order to suppress the reversion of a rubber composition that is capable of being vulcanized and is used for rubber products such as tires and to improve the heat resistance of the rubber composition, there have been known a method including increasing an amount of a vulcanization accelerator to be added to sulfur as a vulcanizing agent, a method including blending a thiuram-type vulcanization accelerator as a vulcanization accelerator, and the like. Furthermore, PERKALINK 900 and Duralink HTS (produced by Flexsys), Vulcuren KA9188 (produced by Bayer AG), and the like are known as cross-linking agents that are capable of generating long chain crosslinks represented as —(CH2)6—S— and the like. It is known that blending such cross-linking agents in the rubber composition makes it possible to suppress the reversion of the rubber composition. However, although such methods have an effect on suppressing the reversion of a natural rubber and an isoprene rubber, they have no effect or little effect on suppressing the reversion of a butadiene rubber.